At the present time, as the material for trunk pipelines for long distance transport of crude oil and natural gas, steel pipe for line pipe of the American Petroleum Institute (API) standard X70 (tensile strength 564 MPa or higher) or higher and up to X80 (tensile strength 620 MPa or higher) has been put into practical use. In recent years, for the purpose of increasing the efficiency of transport of crude oil and natural gas, studies have been made on increasing the internal pressure of pipelines. Along with this, X70 or higher or further X80 or higher high strength steel pipe for line pipe having a tensile strength of 600 MPa or more is being required to be made further greater in thickness.
As opposed to this, the method of using controlled rolling and controlled cooling to make the metal structure fine bainite and produce thick-gauge steel plate excellent in strength and toughness is being produced (for example, Japanese Patent Publication (A) No. 2000-256777, Japanese Patent Publication (A) No. 2004-76101, and Japanese Patent Publication (A) No. 2004-143509). Such conventional X80 or more high strength line pipe has a thickness of at most less than 25 mm. Thick line pipe of 25 mm or more or 30 mm or more is being demanded.
In general, when producing thick-gauge steel plate, at the center part of plate thickness, the rolling by the controlled rolling easily becomes insufficient. Further, it also becomes difficult to secure the cooling speed by the controlled cooling. Further, when producing thick steel pipe, thick-gauge steel plate is formed by an UO process into a pipe shape, then the ends are made to abut against each other and the seams are welded by arc welding. This seam welding results in large heat input if the steel pipe becomes thicker. The weld heat affected zone (called the “HAZ”) becomes coarser in grain size, so the drop in low temperature toughness becomes an important problem.
As technology for improving the low temperature toughness of the HAZ of high strength steel pipe for line pipe, the method of utilizing intragranular transformation to make the structure of the HAZ finer has been proposed (for example, Japanese Patent Publication (A) No. 8-325635, Japanese Patent Publication (A) No. 2001-355039, and Japanese Patent Publication (A) No. 2003-138340). The method proposed in Japanese Patent Publication (A) No. 8-325635 causes the formation of acicular ferrite as nuclei for oxides. The method proposed in Japanese Patent Publication (A) No. 2001-355039 and Japanese Patent Publication (A) No. 2003-138340 forms intragranular bainite using composite inclusions of oxides and sulfides as nuclei.
Utilization of the intragranular bainite is extremely effective for improving the low temperature toughness of the HAZ. However, if the cooling speed falls due to the increased thickness of the steel pipe, the transformation to bainite becomes insufficient, intragranular ferrite is formed, and the strength falls. For this reason, increasing the thickness of high strength steel pipe for line pipe superior in low temperature toughness has been difficult.